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Recently, plasma CO2 hydrogenation to generate valuable products (e.g. CO and CH4, hydrocarbons and oxygenates) has attracted more and more attention. The conversion of carbon dioxide (CO2) and hydrogen (H2) in an atmospheric non-thermal plasma was investigated by a zero-dimensional plasma kinetic model. This paper focuses on the effect of different feed gas composition ratios (H2/CO2 volume ratio) on the plasma CO2 hydrogenation reaction mechanisms. It is found that H2 addition in plasma not only promotes CO2 dissociation but also changes the plasma chemistry, which seems to significantly enhance the electron density and temperature. Besides, larger H2 content in the H2/CO2 mixture is beneficial to obtaining a higher number density of methanol, which is in good agreement with published experimental data. The temporal distributions of abundant radicals, ions and molecule densities are determined under a series of initial hydrogen content. Conversions of inlet H2/CO2 and selections toward CO and CH4 are calculated and presented. The primary reaction channels related to the production and destruction of CO, H, CH2O, CH3OH, CH4 and H2O are determined. Finally, the underlying overall reaction mechanisms regarding the plasma CO2 hydrogenation are analyzed in detail by the chemical reaction flow chart.

In this work, a computational modeling study on the mechanism of the acceleration behavior of a plasma bullet in needle-plane configuration is presented. Above all, in our model, two sub-models of time-dependent plasma dynamics and laminar flow are connected using a oneway coupled method, and both the working gas and the surrounding gas around the plasma jet are assumed to be the same, which are premixed He/N2 gas. The mole fractions of the N2 (NMF) ingredient are set to be 0.01%, 0.1% and 1% in three cases, respectively. It is found that in each case, the plasma bullet accelerates with time to a peak velocity after it exits the nozzle and then decreases until getting to the treated surface, and that the velocity of the plasma bullet increases at each time moment with the peak value changing from 0.72×106 m/s to 0.80×106 m/s but then drops more sharply when the NMF varies from 0.01% to 1%. Besides, the electron impact ionizations of helium neutrals and nitrogen molecules are found to have key influences on the propagation of a plasma bullet instead of the penning ionization.

In this paper, plasma kinetics of CH4/O2 at atmospheric pressure are of interest for applications in plasma chemistry where partial oxidation of methane to oxygenates and syngas are investigated by zero-dimensional global model. Effect of methane mole fraction ranging from 5% to 99% on plasma kinetics is studied in detail.It is found that electron density and electron temperature change periodically with periodic variation of power density pulse. The number densities of free radicals, ions and stable molecules are discussed as a function of simulation time and CH4/O2 molar ratio. Conversions of inlet gases, selectivities and yields of important products, and optimum methane content for methanol and formaldehyde formation are calculated and presented. In addition, CH3 radical is found to be the key intermediate for oxygenates production by analyzing the underlying dominant reaction pathways. Finally, a schematic overview of the transformation relationship between dominant plasma species is also summarized and shown

This paper reports that a plasma generator at atmospheric pressure, which is characterized by double dielectric layers and a stable dielectric-barrier discharge (DBD) excited by a sinusoidal power supply of 30 kHz, successfully generates a low-temperature argon plasma jet. The DBD has a coaxial and cylindrical configuration, with an annular gap of 2.35 mm between the inner and outer quartz tubes. The rotational $(T_{\rm rot})$ and vibrational $(T_{\rm vib})$ temperatures of the DBD are measured by fine structure fitting of the emission bands of UV OH, N2 molecules, and the Boltzmann plot method, and in comparison with the data of jet temperature obtained by an optical fiber thermometer. The obtained experimental results suggest that $T_{\rm rot}$ and jet temperature increase with the peak applied voltage $(V_{p})$ . On the other hand, they are inversely proportional to the argon gas flow rate. In addition, $T_{\rm vib}$ is found to be insensitive to $V_{p}$ and gas flow rate. Furthermore, the correlation between electrical diagnosis and ultrahigh-speed photographs shows that there are two plasma jets at the positive and negative half cycles of the applied voltage. And the temporal and spatial evolution of the DBD inside the quartz nozzle and plasma jet is also elucidated.

In this paper, a two-dimensional axisymmetric fluid model is applied to investigate the streamer discharge characteristics in an atmospheric pressure methane-air plasma jet as a function of methane flow velocities (2.5 m/s and 20 m/s, respectively). Although the streamer ignition and propagation in the dielectric tube are not sensitive to the methane gas flow velocity, the concentration field of methane and air in the mixing layer established by a balance between convective methane flow and back-diffusion of air ambient is crucial for streamer propagation in the gap. As the methane flow velocity is 2.5 m/s, the structure of the streamer head transits from ring-shape into solid disk-shape, while the streamer head always maintains a donut-shaped pattern at high flow velocity of 20 m/s until it impinges on the substrate. At lower gas velocity, the back-diffusion of ambient air into the methane jet is even more pronounced, which causes a larger space charge density at the streamer head, and thus the local electric field near streamer head is greater. Therefore, the overall trend in streamer propagation speed versus methane flow velocity is that the larger the flow velocity, the lower plasma bullet speed. Besides, as the gas flow velocity increases from 2.5 to 20 m/s, less oxygen/nitrogen radical species and charged ions are produced in the streamer discharge, while the produced methane-related particles increase slightly. As for different methane flow velocities, the streamer advances within the methane core.

In this paper, a detailed fluid model is developed for the conversion of methane to methanol with hydrogen peroxide vapor in an atmospheric dielectric barrier discharge. The two-dimensional axisymmetric fluid model is constructed, in which 107 plasma chemical reactions and 28 different species are considered. Our attention is focused on physicochemical mechanism of methanol formation during the complicated chemical reaction processes of reactant molecules dissociation by electron impacting and neutral radical recombination. First, spatial and temporal characteristics of main radicals and ions, such as H, CH3, OH, CH3OH, CH3O, CH2OH, CH4 +, CH3 +, H2O2 +, and H2O+, are presented. It is found that the streamer discharge is sustained by the direct electron-impact ionization of methane molecules. The dominant positive ion flux on to the dielectric surface is methane ion, and its peak value is located at axis. Then, the dominant chemical routes governing the production and loss of CH3OH and OH are discussed in detail. Finally, a schematic overview of dominant plasma reaction pathways for partial oxidation of methane to methanol with hydrogen peroxide vapor in atmospheric dielectric barrier discharge is summarized.

Atmospheric dielectric barrier discharge (DBD) was found to be promising in the context of plasma chemistry, plasma medicine, and plasma-assisted combustion. In this paper, we present a detailed fluid modeling study of abundant radical species produced by a positive streamer in atmospheric dense methane-air DBD. A two-dimensional axisymmetric fluid model is constructed, in which 82 plasma chemical reactions and 30 different species are considered. Spatial and temporal density distributions of dominant radicals and ions are presented. We lay our emphasis on the effect of varying relative permittivity (er = 2, 4.5, and 9) on the streamer dynamics in the plasma column, such as electric field behavior, production, and destruction pathways of dominant radical species. We find that higher relative permittivity promotes propagation of electric field and formation of conduction channel in the plasma column. The streamer discharge is sustained by the direct electron-impact ionization of methane molecule. Furthermo...

Recently, low-temperature atmospheric pressure plasmas have been proposed as a potential type of 'reaction carrier' for the conversion of methane into value-added chemicals. In this paper, the multi-physics field coupling software of COMSOL is used to simulate the detailed discharge characteristics of atmospheric pressure methane-air plasma. A two-dimensional axisymmetric fluid model is constructed, in which 77 plasma chemical reactions and 32 different species are taken into account. The spatial density distributions of dominant charged ions and reactive radical species, such as H, O, CH3, and CH2, are presented, which is due to plasma chemical reactions of methane/air dissociation (or ionization) and reforming of small fragment radical species. The physicochemical mechanisms of methane dissociation and radical species recombination are also discussed and analyzed.

In this paper, an atmospheric pressure argon plasma plume generated by sinusoidal power input of moderate frequency (kHz range) and designed with dual-power electrodes is characterized and studied. Particularly, the effect of driving frequency in the 60–130 kHz range on the argon plasma discharge characteristics is investigated based on a detailed electrical and spectroscopic diagnostics. And, temporal and spatial optical emission spectroscopy is used to measure the plasma parameters, of which the excitation electron temperature is determined by the Boltzmann’s plot method whereas the electron density is estimated using the stark broadening of the hydrogen Balmer Hβ line. It is shown that at constant applied voltage and gas flow rate, the increase of driving frequency in the range of 60–100 kHz exerts no significant influences on discharge parameters. While once the driving frequency exceeds a certain value of about 100 kHz, the discharge becomes intense abruptly and the corresponding discharge parameters increase drastically with the driving frequency. Detailed analysis about the effect of driving frequency on discharge characteristics is presented and two different dominant electron loss mechanisms, namely transport-dominated loss and diffusion-dominated loss, are proposed to account for the distinct effects of the driving frequency on argon discharge characteristics.

Optical emission of the plasma generated on metal hydride target by pulsed laser beam from an Nd:YAG laser was used to investigate the temporal evolution of the line relative intensity, ion (electron) temperature (Te) and electron density (Ne). From relative emission intensities of the atomic and ionic species, the ion temperature using Saha- Boltzmann plots were evaluated. The electron density was determined utilizing the Stark broadening of the Hα-line at 656.27 nm. It was found that numerous lines of double-charged metal ion were visible only at the first 200 ns, and the single-charged metal ion reached the line intensity maximum. The relative intensities of ion line decrease with the increasing time, as opposite to atomic line became more intensive after 1 μs. The max electron density and ion temperature was between 400 ns and 600 ns, then ion temperature decreased from 1.3 eV down to 0.9 eV at the following 4 μs, but the electron density varied so irregularly that further investigation was required in furture.

In this paper, a pulsed-dc CH3OH/Ar plasma jet generated at atmospheric pressure is studied by laser-induced fluorescence (LIF) and optical emission spectroscopy (OES). A gas–liquid bubbler system is proposed to introduce the methanol vapor into the argon gas, and the CH3OH/Ar volume ratio is kept constant at about 0.1%. Discharge occurs in a 6-mm needle-to-ring gap in an atmospheric-pressure CH3OH/Ar mixture. The space-resolved distributions of OH LIF inside and outside the nozzle exhibit distinctly different behaviors. And, different production mechanisms of OH radicals in the needle-to-ring discharge gap and afterglow of plasma jet are discussed. Besides, the optical emission lines of carbonaceous species, such as CH, CN, and C2 radicals, are identified in the CH3OH/Ar plasma jet. Finally, the influences of operating parameters (applied voltage magnitude, pulse frequency, pulsewidth) on the OH radical density are also presented and analyzed.

Recently, C/H/Ar plasma discharges found enormous potential and possibility in carbonaceous compounds conversion and production. In this work, a pulsed-dc CH3OH/Ar plasma jet generated at atmospheric pressure is investigated by means of optical and electrical diagnosis concerning the variation of its basic parameters, absolute concentration of OH radicals, and plasma temperature with different CH3OH/Ar volume ratios, in the core region of discharge with needle-to-ring electrode configuration. The voltage–current characteristics are also measured at different CH3OH/Ar ratios. Laser-induced fluorescence (LIF) results here show that only small amounts of added methanol vapor to argon plasma (about 0.05% CH3OH/Ar volume ratio) is favorable for the production of OH radicals. The optical emission lines of CH, CN, and C2 radicals have been detected in the CH3OH/Ar plasma. And, the plasma temperatures increase with successive amount of added methanol vapor to the growth plasma. Moreover, qualitative discussions are presented regarding the mechanisms for methanol dissociation and effect of the CH3OH component on the Ar plasma discharge at atmospheric pressure.

In this paper, a two-dimensional coupled model of neutral gas flow and plasma dynamics is presented to explain the gas flow dependence of discharge characteristics in helium plasma needle at atmospherics pressure. The diffusional mixing layer between the helium jet core and the ambient air has a moderate effect on the streamer propagation. The obtained simulation results present that the streamer shows the ring-shaped emission profile at a moderate gas flow rate. The key chemical reactions which drive the streamer propagation are electron-impact ionization of helium neutral, nitrogen and oxygen molecules. At a moderate gas flow rate of 0.5 slm, a significant increase in propagation velocity of the streamer is observed due to appropriate quantity of impurities air diffuse into the helium. Besides, when the gas flow rate is below 0.35 slm, the radial density of ground-state atomic oxygen peaks along the axis of symmetry. However, when the gas flow rate is above 0.5 slm, a ring-shaped density distribution appears. The peak density is on the order of 1020 m−3 at 10 ns in our work.

A numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet is presented. The generation and loss mechanisms of the OH radicals in a positive half-cycle of the applied voltage are studied and discussed. It is found that the peak OH density increases with water content in air (varying from 0% to 1%) and reaches 6.3×1018 m−3 when the water content is 1%. Besides, as the water content increases from 0.01% to 1%, the space-averaged reaction rate of three-body recombination increases dramatically and is comparable to those of main OH generation reactions.

A two-dimensional coupled model of neutral gas flow and plasma dynamics is employed to investigate the streamer dynamics in a helium plasma needle at atmospheric pressure. A parametric study of the streamer propagation as a function of needle tip curvature radius and helium gas flow rate is presented. The key chemical reactions at the He/air mixing layer which drive the streamer propagation are the direct ionization via collision with electrons, the Penning effect being not so crucial. With increasing the gas flow rate from 0.2 standard liter per minute (SLM) to 0.8 SLM, however, the emissions resulting from reactive oxygen and nitrogen species change from a solid circle to a hollow profile and the average streamer propagation velocity decreases. Air impurities (backdiffusion from ambient air) in the helium jet result in a significant increase in the streamer propagation velocity. Besides, with decreasing the tip curvature radiusfrom 200 μm to 100 μm, the electron avalanche process around the near-tip region is more pronounced. However, the spatially resolved plasma parameters distributions (electron, helium metastables, ground state atomic oxygen, etc.) remain almost the same, except that around the near-tip region where their peak values are more than doubled.

Two Stark broadening parameters including FWHM (full width at half maximum) and FWHA (full width at half area) of isotope hydrogen alpha lines are simultaneously introduced to determine the electron density of a pulsed vacuum arc jet. To estimate the gas temperature, the rotational temperature of the C2 Swan system is fit to 2500 ± 100 K. A modified Boltzmann-plot method with bi-factor is introduced to determine the modified electron temperature. The comparison between results of atomic and ionic lines indicates the jet is in partial local thermodynamic equilibrium and the electron temperature is close to 13 000 ± 400 K. Based on the computational results of Gig-Card calculation, a simple and precise interpolation algorithm for the discrete-points tables can be constructed to obtain the traditional ne-Te diagnostic maps of two Stark broadening parameters. The results from FWHA formula by the direct use of FWHM = FWHA and these from the diagnostic map are different. It can be attributed to the imprecise FW...